(1) Field of the Invention
The present invention relates to switching power supply devices having a constant voltage characteristic suitable to supply an output voltage with high accuracy.
(2) Description of the Related Art
Most of switching power supply devices are required to have a constant voltage characteristic suitable to supply a stable output voltage, regardless of a state of a load and a value of an output current. An electrically isolated switching power supply device equipped with a transformer, however, has an output voltage generating portion and a switching element controlling portion both isolated. Thus, these isolated portions make it difficult for the switching power supply device to carry out control in order to achieve the constant voltage characteristic. In view of such a difficulty, techniques to obtain the constant voltage characteristic have been studied.
As the most typical technique, for example, Patent Reference 1 (Japanese Unexamined Patent Application Publication No. 2007-116890) describes a technique to control a constant voltage used for a switching power supply device. The technique involves the following operations: a shunt regulator placed on the secondary side detects an output voltage; then, the information on the output voltage is transferred to the primary side by a photocoupler; and consequently, the information is transferred to a control circuit. Regardless of the output current value, the above operations allow the control circuit to control an output voltage to be constant by adjusting energy supplied through switching performed by the switching element.
As this example shows, the following three functions are necessary in order to achieve a constant voltage characteristic in an isolated power supply:
(i) a part and a method for detecting an output voltage;
(ii) a method for transferring a signal in an isolated part; and
(iii) a part and a method for adjusting energy supply so as to make an output voltage constant.
An output power is the product of an output voltage and an output current. In order to make the output voltage constant while the value of the output current is changing, energy supply needs to be adjusted. To adjust the energy, the output voltage needs to be detected. In order to prevent the detected output voltage from changing, the supplied energy needs to be adjusted. Since a signal needs to be transferred through the isolated part, as described above, the previously mentioned three functions are required. In this example, the shunt regulator, the photocoupler, and the control circuit for the switching element respectively exercise the functions (i), (ii), and (iii).
This technique, however, requires an expensive photocoupler. Demanded in the market instead is power supplies having no photocoupler in order to reduce the cost.
In order to meet the demand of the market, Patent Reference 2 (Japanese Unexamined Patent Application Publication No. 07-170731) introduces a technique to achieve a constant voltage characteristic without a photocoupler. This technique takes advantage of a voltage generated, in proportion to an output voltage, on an auxiliary winding provided in a transformer. Based on the generated voltage, this technique controls a switching element and achieves the constant voltage characteristic.
In the technique described in Patent Reference 2, the auxiliary winding generating a voltage in proportion to an output voltage exercises the function (i). The auxiliary winding can also exercise the function (ii), working with a secondary winding placed in the transformer. The function (iii) is exercised by a pulse width modulation (PWM) control integrated circuit (IC).
This technique, however, faces a following problem: a spike voltage occurs in a voltage on the auxiliary winding immediately after the switching element is turned off. This generated spike voltage deteriorates the proportionality between the output voltage and the auxiliary winding voltage. In addition, the spike voltage changes as an output power changes. Thus, the technique has a disadvantage in that accuracy of the output voltage deteriorates when the output power changes. In other words, the technique likely shows deterioration of the constant voltage characteristic due to poor accuracy in detecting an output voltage described in the function (i).
Then, formulated was another technique which does not require a photocoupler. Patent Reference 3 (Japanese Unexamined Patent Application Publication No. 2004-229437) describes the technique. In this technique, focused is a secondary-side conduction time period T2on representing a period during which a current flows into a secondary winding. This technique employs a relationship determined among the time period T2on, an output voltage, and a peak value of a primary current waveform. Here, the primary current is a current flowing into a switching element. Specifically, the technique changes the peak value of the primary current waveform so that the relationship is determined with the output voltage remained constant. Accordingly, a constant voltage characteristic is achieved.
In the technique described in Patent Reference 3, the above functions (i), (ii), and (iii) are exercised as follows: the function (i) is exercised in the secondary-side conduction time period T2on; the function (ii) is exercised through detection of the secondary-side conduction time period T2on using a voltage appearing on either a primary winding of a transformer or an auxiliary winding of the transformer; the function (iii) is exercised through a change of the peak value of the primary current waveform.
Specifically described here is detection of the output voltage; namely, the function (i). The technique employs the secondary-side conduction time period T2on to detect the output voltage, which is based on Equation 1 below.VO=(Ns/Np)×(Lp×Ipp/T2on)−VF  (Equation 1)
Here, VO is an output voltage, Ns is the number of turns of a secondary winding, Np is the number of turns of the primary winding, Lp is an inductance value of the primary winding, VF is a forward voltage of a secondary-side rectification diode, T2on is the secondary-side conduction time period, and Ipp is a primary current peak value.
Since Equation 1 holds, if the secondary-side conduction time period T2on can be detected, the output voltage VO can be estimated in the case where Ns, Np, Lp, Ipp, and VF are definite. Premised on the basis that Ns, Np, and Lp are constant and definite, this example involves executing control to cause Ipp and T2on to proportionally change. Accordingly, detection of the output voltage VO and an adjustment of supplied energy are executed through a change of Ipp.